Erectile Dysfunction (ED) is a repeated inability to achieve or maintain a penile erection for sexual intercourse. It is estimated that about 30 million men in the United States and 150 million men worldwide experience chronic erectile dysfunction. Additionally, studies have shown that about half of the men between the ages of 40 and 70 have erectile dysfunction to some degree. The prevalence of erectile dysfunction is illustrated by the fact that Viagra, an ED drug manufactured by Pfizer, has been prescribed to more than 35 million men worldwide as of 2011.
The human penis consists of three erectile bodies: two corpora cavernosa chambers and one corpus spongiosum chamber. Together these three chambers make up the expandable erectile tissues along the length of the penis which fill with blood during penile erection. The two corpora cavernosa lie along the penis shaft, from the pubic bones to the head of the penis, where they join. These formations are made of a sponge-like tissue containing irregular blood-filled spaces lined by endothelium and separated by trabeculae of smooth muscle fibers with an extracellular matrix of which the main components are collagens, elastic fibers, proteoglycans/glycoseaminoglycans, in addition to numerous unmyelinated and preterminal autonomic nerves.
The importance of corporal cavernosal smooth muscle cells (SMCs) in potency is well-established. Normal smooth muscle content and function are necessary for the initiation and maintenance of erection. Additionally, elastin and collagen fibers are also important penile constituents. Collagen is a key structural protein in tissues subjecting to stretching forces. Collagen provides structural integrity that allows the cavernosum to withstand pressure increase during erection. Some researchers argue that without this rigid collagen network, the penis would maintain a flaccid form.
Erectile dysfunction can be caused by a number of factors, including physical, psychological, neurological, vascular and endocrinal issues. Reduced blood flow to the penis and nerve damage are the most common causes of erectile dysfunction. Underlying causes include, but are not limited to, vascular disease, diabetes, drugs, hormone imbalance, neurological causes, pelvic trauma in surgery, Peyronie's Disease and venous leak.
Currently, the most common method for treating erectile dysfunction is through oral inhibitors of phosphodiesterase V (PDE-5), including the medications: Sildenafil (Viagra®), Vardenafil (Lavetra®) and Tadalafil (Cialis®). However, numerous patients remain unresponsive to treatment, do not tolerate the adverse effects associated with the treatment, or are ineligible for treatment. Thus, there remains a need for a safe and effective treatment of erectile dysfunction which can be used without the side effects associated with the use of PDE-5 inhibitors.